The subject matter described herein relates generally to reducing noise generated by an airfoil, and more specifically, to devices and methods for reducing noise by heating the boundary layer at the trailing edge of an airfoil.
Generally, a wind turbine includes a rotor that includes a rotatable hub assembly having multiple blades coupled thereto. The blades transform wind energy into a mechanical rotational torque that drives one or more generators via the rotor. Some known wind turbine blades may generate considerable noise during operation of the wind turbine. As a consequence, local authorities having the responsibility for granting permission for installing wind turbines may refuse to allow installation due to the noise. Alternatively, the wind turbine installation may be authorized with set maximum sound pressure levels that cannot be exceeded. For example, in some locations, issuance of installation permits for wind turbines is based on the environmental noise impact affected or potentially affected by the wind turbine. A wind turbine may be forced to run in a noise-reduced operating mode due to the noise made by the wind turbine or turbines, which in turn may decrease the annual energy production of the wind turbine.
Generally, there are two primary noise source categories on a wind turbine. These include mechanical noise, such as vibrations in the drive train and gear noise, and aerodynamic noise, which is due to aerodynamic processes on the blades. Mechanical noise generally can be reduced using known techniques to dampen or isolate mechanical vibrations in the wind turbine or by employing sound absorbing materials. Aerodynamic noise is often more difficult to reduce, and is considered to be the dominant noise source on at least some known wind turbines.
Aerodynamic noise can be divided into two main general groups. These groups include airfoil self-noise, due to interaction of air flow with the blades, and turbulent inflow noise, due to scattering of turbulent airflow fluctuations by the blades. Airfoil self-noise is further divided into various noise mechanisms, one such example being trailing edge noise. Trailing edge noise is caused by the interaction of turbulence in the boundary layer with the blade trailing edge.
The boundary layer is a very thin sheet of air lying over the surface of the blade that tends to adhere to the blade. As the blade moves, air in the boundary layer region near the leading edge flows smoothly over the streamlined shape of the blade generating a laminar flow layer. As the air continues to flow further along the chord of the blade, the thickness of this laminar flow boundary layer increases due to friction with the blade. At some distance along the chord of the blade a turbulent layer begins to form over the laminar layer. The thickness of the turbulent layer increases and the thickness of the laminar layer decreases as the air flows further along the blade. The onset of transition flow, where the boundary layer changes from laminar to turbulent is called the “transition point,” and is where drag due to skin fiction becomes relatively high. This transition point tends to move forward on the chord of the blade as the speed and angle of attack of the blade increases, resulting in more drag and more noise-causing turbulence.